Heretofore, it has been known that in order to obtain the lowest aging rate in an oscillator, the drive current through the resonator had to be low. Typically, the drive current has been set to a value between 0.1 and 1.5 ma. A higher drive current has been known to result in higher aging rates. The reasons for the high aging rates are not fully understood. One reason that can account for the higher aging rates is that, at high drive currents, the resonators become more nonlinear, that is, slight changes in drive current can cause significant frequency changes. Another reason is that at higher drive currents, particles and molecules that are adsorbed onto the resonator's surfaces experience larger accelerations. Those larger accelerations increase the probabilities of desorption. Particles desorbing result in frequency jumps; molecules desorbing faster add a positive contribution to the aging rate.
On the other hand, the difficulty with the continuous low drive current as currently employed is that, at low drive, the aging rate of the oscillator decreases at a very slow rate that is unacceptable for many applications as for example, as an oscillator for a satellite capable of operating continuously for many years.
Heretofore, the only accelerated aging tests that have been applied to quartz resonators and oscillators have consisted of operating the resonators and oscillators at elevated temperatures, e.g. one to three weeks at 125.degree. C., or the thermal step stress method described by S. H. Olster et al, in an article entitled "A Monolithic Crystal Filter Design for Manufacture and Device Quality", on pages 105-112 of the Proceedings of the 29th Annual Symposium of Frequency Control, 1975. Such accelerated aging has been useful for process control, but not for improving the long term aging at the normal operating temperatures.